caffe.proto: do some minor cleanup (fix comments, alphabetization)
Add Net::Reset and Layer::Reset methods.

Net::Reset calls Layer::Reset on all its layers.
Layer::Reset currently has a trivial implementation only; should be
impelmented for data layers.
Solver: call Reset on the test nets if specified by a new
SolverParameter field: test_net_reset.
Implement DataLayer::Reset()
Implement HDF5DataLayer::Reset()
Implement ImageDataLayer::Reset()
Implement DummyDataLayer::Reset() -- die on non-constant data.
Die on WindowDataLayer::Reset -- always uses RNG.